Hardware Reference
In-Depth Information
Figure 2.14
Simple view of the USB interface
Figure 2.15
Layers in USB architecture (note the apparent peer-peer
logical interconnection)
device. The
Function layer
provides additional capabilities to the host via an
appropriate matched client software layer. The USB Device and Function layers
each have a view of
logical
communication within their layer that actually uses
the USB Bus interface layer to accomplish data transfer. What is important
about this model is the apparent
peer-to-peer connectivity
that it provides!
Devices on the Universal Serial Bus are physically connected to the host via
a tiered star topology. Figure 2.16 shows the topology of a typical USB arrange-
ment. Note that host effectively incorporates its own embedded hub, called the
Root Hub
. The Root Hub, in turn, provides one or more attachment points.
Multiple functions may be packaged together in what appears to be a single
physical device. For example, a keyboard and a trackball might be combined in
a single package. Inside the package, the individual functions are permanently
attached to a hub and it is the internal hub that is connected to the USB. When
multiple functions are combined with a hub in a single package, they are referred
to as a compound device. From the host's perspective, a compound device is
the same as a separate hub with multiple functions attached.
